An inflator for inflating an inflatable vehicle occupant protection device includes a quantity of a stored gas and a body of combustible material stored in an inflator housing. An igniter is actuatable to ignite the body of combustible material. As the body of combustible material burns, the combustion products heat the stored gas. The heated stored gas and the combustion products form an inflation fluid for inflating the vehicle occupant protection device. Another inflator includes a stored inert gas and a stored combustible gas, such as hydrogen. An igniter ignites the combustible gas, which heats the stored inert gas.
An inflator housing can be formed from steel. The steel used to form the inflator housing must have sufficient tensile strength to store the compressed gas at an elevated pressure. The steel must also have a toughness sufficient to prevent brittle fracture of the inflator housing when the inflator housing is subjected to temperatures down to about −40° C.